1. Field of the Invention
The present invention relates generally to a system for sensing the presence of certain chemical species in vapor form in a gaseous mixture and, more particularly, to the use of selectively permeable stabilized liquid membranes in combination with general purpose semiconductor or catalytic gas sensors to produce a selectively responsive analytic tool. That is, the detector sensing system of the invention is one in which the selectivity is enhanced by the permeability of the membrane or "permselectivity" rather than being based solely on particular selective adsorption or ion selectivity characteristics of the sensing element.
2. Description of the Prior Art
Relatively general purpose gas sensing elements and devices of various types have been in existence for some time. These include relatively non-selective or generic, stabilized solid state devices such as those exhibiting catalytic action to promote the oxidation of organic vapors at elevated temperatures. A semiconductor gas sensor, for example, composed mainly of tin dioxide (SnO.sub.2) responds to reducing gases, including organic vapors, generally. A catalytic sensor such as a platinum wire or a platinum thin film configuration which might, for example, take the form of a microscopic bridge structure such as that structure shown in U.S. Pat. No. 4,624,137 is also in this category.
Such sensors are relatively inexpensive and perform admirably in many applications. However, sensitivity to common atmospheric variables has heretofore limited the scope of effective utility for these devices. SnO.sub.2, for example, is highly sensitive to changes in the level of relative humidity in the sensing environment. In addition, all such sensors are sensitive to flow and changes in the velocity of the sensed vapors. These and other phenomena produce changes in sensor output, thereby introducing error in the readings.
Of course, generic sensors inherently lack selectivity. This lack of selectivity may not be a primary drawback in certain applications where, for instance, a single species is being monitored in a known mixture. However, many gases can produce sensor responses of like or even order and, therefore, interference possibilities have always required attention.
Thus, definite needs exist in the art for sensing systems which can take advantage of the sensitivity and relatively low cost of relatively non-selective generic sensors such as tin oxide notwithstanding common interfering species in the sample atmosphere.